Both your eyes meet. A sense of familiariy ensues, you'v seen each other few many times at the gym, have cycled on stationary bicycles next to each other, have ran couple of miles on treadmills next to each other.
You contemplate between smiling and and mouthing a 'Hi'. A half smile begins to emerges out of doubt, she looks at you for a moment, then turns her glare elsewhere and walks away. You are thinking, "damn !" and walk away, hoping your half smile emerged only after her glare shifted elsewhere.